In my personal canon that Marcie is the same character as the Marcie from Peanuts. And if we’re importing characters, then Mandy could possibly, at a stretch, be a grown-up Peppermint Patty. Their poses in this wallpaper indicate that Marcie has grown to shun Sir in her independent, skateboarding college years, and is supported in her resentment by this ‘Sierra,’ but Peppermint Mandy doesn’t care, or is at least putting up a good front.